Signal filtering within a radio receiver is an essential part of signal processing to decode the transmitted information. Often, filtering is done by designing active filters based on LC ladder filters, as this topology is known to have very low sensitivity to errors in its component values. The main problem with trying to design LC filters today is that they are largely incompatible with present integration techniques. This is mainly because of the unintegratability of the inductor (L) component. It is known that an inductor can be simulated by an electronic component called a gyrator. Gyrators are made of transistors, bipolar or MOS or both, and a capacitor. All these components are fully compatible with today's integration technologies.
A further requirement of these signal filters is that they be programmable (tunable) in their bandwidth. A programmable bandwidth is achievable by varying the transconductance of the Operational Transconductance Amplifiers (OTAs) used in the design of gyrators. It is known that filter bandwidth is directly proportional to the OTA transconductance and inversely proportional to the capacitance. It is therefore desired to build an inductor with OTAs in silicon with a programmable transconductance.
Previously, designers were able to achieve programmability by using topologies with translinear stages. A detailed explanation of the workings of one such topology can be found in "A New Wide Band Amplifier Technique" by Barrie Gilbert which was published in the IEEE Journal of Solid State Circuits, vol. SC-3, #4, Dec. 1968.
As illustrated in the above mentioned paper, the translinear stage consists of a differential pair with an active current mirror. A problem with such a translinear stage is that a large number of devices contribute significant device noise to the overall circuit noise thereby increasing the noise floor. This significantly affects the dynamic range (DR), which is defined as the ratio of the maximum usable signal to the minimum usable signal (or noise floor plus distortion). Since supply voltages above 5V have been used in designing the circuits of the translinear stage, designers have able to achieve an output swing of a few volts. This large voltage output swing allowed an acceptable dynamic range even though the noise floor was high. However, when the supply voltage drops significantly below 5 volt, a large portion of the output signal swing is lost. Depending on the supply voltage, this loss could be up to 65% of the output swing. Thus, the DR quantity severely decreases as a natural consequence.
It is therefore desired to have a programmable OTA that could operate at low supply voltages without a significant increase in the total noise floor.